[Core Graphics] Disambiguate the CGContext PDF endPage

This changes how we import CGPDFContextEndPage to be
CGContext.endPDFPage(), instead of .endPage() which conflicts with
another CGContext function. Also does .beginPDFPage() for consistency,
which is consistent with e.g. drawPDFPage().

Author: milseman

apinotes/CoreGraphics.apinotes

@@ -386,9 +386,9 @@ Functions:
 - Name: CGPDFContextClose
   SwiftName: CGContext.closePDF(self:)
 - Name: CGPDFContextBeginPage
-  SwiftName: CGContext.beginPage(self:_:)
+  SwiftName: CGContext.beginPDFPage(self:_:)
 - Name: CGPDFContextEndPage
-  SwiftName: CGContext.endPage(self:)
+  SwiftName: CGContext.endPDFPage(self:)
 - Name: CGPDFContextAddDocumentMetadata
   SwiftName: CGContext.addDocumentMetadata(self:_:)
 - Name: CGPDFContextSetURLForRect